The present invention relates to a driver circuit for soft turn-on of a power element connected to an inductive load. The invention relates more particularly, but not exclusively, to a driver circuit for soft turn-on of an IGBT power transistor connected in an electronic ignition system for automotive applications.
Inductive loads for coils are associated with a power element for driving them, and in particular, for turning them on/off. A driver circuit is, therefore, associated with the power element. In an automotive electronic ignition system, an IGBT power transistor is connected to a spark plug, for example.
FIG. 1 shows schematically a final stage 1 of a conventional electronic ignition system. In particular, the final stage 1 is connected between a first supply voltage reference, e.g., a battery VB, and a second voltage reference, e.g., ground GND. The final stage 1 comprises an electronic switch 2 connected between an inductance L1 of the primary windings of a coil 3 that has a corresponding inductance L2 of the secondary windings connected to an output terminal OUT1 of the final stage 1. A load 4, e.g., a spark plug in automotive applications, is connected to the output terminal OUT1 of the final stage 1.
The electronic switch 2 is also connected to an input terminal IN1 of the final stage 1. By providing a logic input signal Vin1 at a logic high to the input terminal IN1, the electronic switch 2 enables a charge current Iloadl to flow through so that energy is stored in the primary winding inductance L1. In a complementary way, when the logic input signal Vin goes low, the electronic switch 2 opens to generate an overvoltage value Vc1 to a terminal of the primary winding inductance L1.
This overvoltage Vc1 generates a trigger voltage Vsec1 across the secondary winding inductance L2, dependent on the turn ratio of the primary and secondary windings in the coil 3. In the exemplary load under consideration, the ratio of turns of the coil 3 is selected to generate a trigger voltage Vsec1 of thousands of volts, so that a spark can be generated across the spark plug gap over the output terminal OUT1.
Shown schematically in FIG. 2 are typical waveforms of a load turn-on in a conventional electronic ignition system whose final stage comprises an electronic switch of the kind described in relation to FIG. 1. In such electronic ignition systems, an error condition may be entered as a consequence of an undesirable trigger voltage overshoot dVsec1/dt across the secondary winding inductance L2. For example, during a turn-on phase of the electronic ignition system that comprises the final stage 1 and spark plug 4, a voltage variation dVc1/dt across the primary winding inductance L1 may produce a sufficient voltage variation dVsec1/dt across the secondary winding inductance L2 to generate a spark at the spark plug 4.
FIG. 3 shows schematically a waveform related to a conventional turn-on procedure in a conventional electronic ignition system employing an IGBT transistor 2 for an electronic switch. It is known that, upon the occurrence of a negative voltage variation dVc1/dt, the trigger voltage Vsec1 may attain a high value of 1,000 Volts, which would result in a spark being generated at the spark plug 4.
For this reason, many specifications regarding automotive applications set a maximum of 500 to 600 Volts for the trigger voltage Vsec1 to the secondary winding inductance L2 during those phases when no spark is required. It has been known to use for this purpose a soft turn-on procedure for the electronic switch 2, coincidently with turning on the electronic ignition system that contains the switch. This is done to minimize the effects of a first negative variation dVc/dt of the voltage to the primary winding inductance L1, and of the consequent overshoot dVsec1/dt of the trigger voltage Vsec1 to the secondary winding inductance L2. In particular, for a softer turn-on of the power device in the electronic switch 2, a capacitor is connected between a collector terminal of the power device and its control terminal.
This first prior art approach requires that a high voltage element (HW) be provided external the ignition system. This approach reflects on the overall cost of the system. It also has a major disadvantage in that the power device turn-off is slowed during the phase of generating a spark to the load, resulting in wasted energy from the primary winding inductance L1 to the secondary winding inductance L2.
Shown schematically in FIG. 4 are waveforms of internal signals of an IGBT transistor that is driven with a constant gate current Ig. Upon a gate voltage Vg of the IGBT transistor reaching a threshold voltage value Vth of the transistor, the transistor becomes conductive. This allows a collector current Ic to go through and the load device to be turned on. During this phase, the IGBT transistor will increase the initial slope of the gate voltage Vg by the Miller effect.
As is well known, this Miller effect is due to the voltage evolution at a collector terminal of the IGBT transistor, and disappears as the negative variation dVc/dt of the collector voltage passes away. In other words, the IGBT transistor exhibits a large equivalent capacitance due to the Miller effect, in parallel with its intrinsic gate-source capacitance Cgs.
A single way of influencing the turn-on phase of the IGBT transistor is that of driving its gate terminal with a very small gate current Ig, so as to further extend the duration of charge delivery to the total equivalent capacitance Cgs*, which is subject to the Miller effect, and change the slope of a collector terminal current Ic. This prior driving procedure is unrelated to the behavior of the electronic voltage reference, for example, a high voltage connected to the IGBT transistor, which would be a problem for circuitry that is powered from the gate terminal of the IGBT transistor.
A limited driver circuit, as shown generally at 5 in FIG. 5 in schematic form, may also be used. In particular, the limited driver circuit 5 is connected between a first voltage reference, such as a supply voltage VDD, and a second voltage reference, such as ground GND, and is connected to a gate terminal G6 and an emitter terminal E6 of an IGBT transistor 6.
The IGBT transistor 6 has a collector terminal C6 connected to a load device 9, which in turn is connected to the supply voltage reference VDD. The limited driver circuit 5 further comprises a generator 7 for generating a drive current Idriv, and is connected between the supply voltage reference VDD and the gate terminal G6 of the IGBT transistor 6. A limiter circuit 8 is connected between the gate terminal G6 and the emitter terminal E6 of the IGBT transistor 6. The emitter terminal E6 is connected to ground GND.
The drive current Idriv is thus separated as a first current portion (current Ilim) and a second current portion (Ig). The current Ilim is absorbed by the limiter circuit 8. The gate current Ig is for the gate terminal G6 of the IGBT transistor 6. The total equivalent capacitance Cgs* of the IGBT transistor 6 is, therefore, charged with a gate current Ig given as:
Ig=Idrivxe2x88x92Ilim.
To obtain a narrowly limited value of the gate current Ig, the currents Idriv and Ilim must have comparable values. This condition is, however, made critical by process temperature spreads in the ignition system that comprises the limited driver circuit 5, which can cause a current balance such that the load device 9 cannot be turned on.
In view of the foregoing background, an object of the present invention is to provide a driver circuit for soft turn-on of a power element to prevent the occurrence of voltage overshooting at a load connected to the power element, and thus overcome the shortcomings of the driver circuits of the prior art.
This and other objects, advantages and features of the present invention are provided by a pair of current generators for driving the power element, and generating different current values in parallel with each other. The driver circuit charges the gate capacitor with a small current only after the power element initiates a turn-on phase.
Based on this principle, the technical problem is addressed by a driver circuit comprising at least a second current generator connected, in parallel with a first current generator, between the voltage reference and the output terminal to provide the control terminal with a second charge current dependent on a voltage value present at the input terminal. The input terminal is connected to a conduction terminal of the power element.